This invention relates to a torque transmitting assembly for a wheel drive train and, in particular, an assembly suitable for the wheels of large, off-highway vehicles.
Large, off-highway trucks for mining and quarrying operations are of course well known and many of these trucks have capacities in excess of one hundred tons. These vehicles must often be able to negotiate steep inclines such as where they are used in open pit mines, these pits often being very deep indeed. Many of these large trucks utilize electric wheel drive systems mounted within the rear axle. Vehicles driven in this manner generally have two electrical propulsion systems, one located on each side of the axle with the central housing portion of the axle providing access for such features as the electrical circuitry. The primary power source of these trucks usually consists of a large internal combustion engine or the equivalent and this engine drives a large capacity electrical generator. Generally, a substantial gear reduction system is operatively connected between the electric motor for each rear wheel and the rear wheel is often a dual wheel.
Electrically or hydraulically propelled vehicles are more easily adapted to design configurations when the payload of such vehicles approaches and exceeds approximately one hundred tons. This is not to say that mechanical or equivalent drives are not utilized. Rather, within the usual basic design criteria, there are some space advantages in utilizing either electric or hydraulic propulsion means for such vehicles.
Presently, the large rear drive wheels of these vehicles which are driven by an electric motor in each wheel have a gear reduction system that includes a torque tube assembly for transmitting torque from the planetary gears of the system to the actual wheel. This known torque tube assembly has basically three components: namely, a ring gear (hereinafter called the internal gear member), a barrel or cylindrical member (hereinafter called the first torque transmitting member), and a base plate of generally cylindrical form (hereinafter called the second torque transmitting member). These three components of the known torque tube assembly are generally welded together for use in the overall wheel assembly.
The known torque tube assembly has a number of disadvantages primarily due to the fact that in its assembled state it comprises a single rigid, welded member. Any welding operation will of course cause distortions in the welded assembly because of the heat created during welding. Also, generally speaking, any welded assembly has geometric problems (such as cracks and undercutting), changes that have occurred in the heat affected zone, and the build up of residual stresses in the assembly. A further disadvantage of a one piece welded torque tube assembly as that the various components must be aligned and held very carefully when the welding procedure is being carried out. It will be appreciated that a torque tube assembly for a very large truck wheel will itself be of considerable size. Thus, a high degree of accuracy is required in the welding operation and on going processes must be used to produce the finished welded torque tube assembly in order to provide for relief of the residual stresses developed during welding.
Another disadvantage of the one piece welded torque tube assembly is that it creates an overall wheel rim assembly that tends to be extremely rigid. Obviously in the case of a wheel rim assembly that is used in the rear axle of the aforementioned large trucks, tremendous external and operating forces will be exerted on the wheel rim assembly during even normal truck operation and these forces are absorbed by the known torque assembly itself to some extent. Thus, not only must the torque tube assembly be able to absorb extremely large torsional forces but it must also bear at least some of the load bearing forces transmitted to the overall wheel rim assembly. In the known welded torque tube assemblies, distortion and even failure can occur under the great torsional and bending forces. Moreover, high stress concentrations generally occur in the welded areas of the tube assembly. Actual field experience has shown that fatigue failures have occurred in the weld areas.
Another major disadvantage in many of the known, welded torque tube assemblies arises from the necessity of providing access openings in the first cylindrical torque transmitting member (i.e. the barrel portion of the assembly) in order that an internal planetary gear bearing located within this member can be suitably mounted and adjusted for operation thereof. There is no other practical way of mounting this particular bearing where a one piece torque tube assembly is employed. The areas cut out of the first torque transmitting member to provide these openings develop high stress concentrations during use of the vehicle. Experience has again shown that fatigue failures occur in the areas of these openings.
Accordingly it is an object of the invention to provide a novel torque transmitting assembly for a wheel drive train which is better able to withstand great torsional and bending forces created during use of the wheel assembly.
It is a further object of the invention to provide such a torque transmitting assembly which has a longer operational life because of the use of gear means rather than welds to transmit torque between the parts of the assembly.
The present torque transmitting assembly overcomes many of the disadvantages of the prior art by providing a torque transmitting assembly that preferably consists of three separate parts rather than one and thus the assembly of the present invention is not rigid and does not require welding. Because of the flexibility of the connections between the three parts, there is no need for the assembly of the present invention to absorb large bending forces and the large torsional forces are transmitted between the members by means of a gear arrangement. Due to the gear arrangement of the three members which make up the assembly, large torsional forces are distributed evenly about the members, thus avoiding any stress concentration.
An additional object of the torque transmitting assembly of the present invention is to provide such an assembly which can be incorporated into existing wheel rim assemblies by simply substituting it for the original welded, one piece torque tube assembly.